Screw for extruder and method of manufacturing the extruderer, and rubber member for tire and method of manufacturing the rubber member

ABSTRACT

A screw  12  for an extruder  10  rotatably supported on the cylinder of the extruder and carrying roll-heated rubber material supplied from a hopper port  13   s  provided at the rear part of the cylinder  11,  wherein the height of the portion of the flight portion  12   z  of the screw  12  located below the hopper port  13   s  is set lower than the height of the flight portion  12   a  on the downstream side of the screw  12,  whereby the pulsation of extruded matter can be reduced while maintaining a high discharge rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a screw for use in extruders and,particularly, to the structure of a screw for reducing the pulsation ofan extruded product and a production process therefor.

2. Description of the Prior Art

In the prior art, to manufacture a rubber member which requires apredetermined sectional form, such as a tire tread or a side wall, anextruder comprising a screw is generally used. For instance, in the stepof extruding a tire tread, a kneaded and blended rubber material fortreads is heated with a warming mill to be softened, supplied to thehopper 51 of an extruder 50 shown in FIG. 3, carried to the end of acylinder 53 and extruded from the nozzle 55 of a head 54 attached to theend of the cylinder 53 by a screw 52 to be molded into a predeterminedsectional form, cooled and cut to a required length.

However, as the above extruder 50 generally has a short length (L/D),the shape of the rubber material to be molded is easily affected by therevolution of the screw 52. Therefore, when the extrusion rate (deliveryrate) is increased by changing the structure of the screw, pulsationcaused by delivery becomes large.

For example, when a high-delivery type screw 62 having a double-threadstructure as a whole and a single-thread structure only on the hopperport 51 s side is used as shown in FIG. 4, the delivery rate becomeshigher than in the prior art. However, as all the rubber materialsupplied by the above single-thread structure portion is forced into thecylinder 53, a change in pressure applied from the rubber material tothe flight portion 62F of the screw 62 become large in the vicinity ofthe hopper port 51 s. Therefore, the pulsation of the delivered extrudedproduct A becomes large, thereby increasing a gauge fluctuation.

Particularly when the extruded product is a tread, the above gaugefluctuation exerts a bad influence upon the uniformity and balance oftires. Therefore, the development of a screw capable of suppressingpulsation even when the extrusion rate is increased is desired.

It is an object of the present invention which has been made in view ofthe above problem of the prior art to provide a screw for extruderscapable of reducing the pulsation of an extruded product while a highdelivery rate is maintained.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda screw for use in an extruder for carrying a rubber material suppliedfrom a hopper port at the rear of the cylinder of the extruder by thescrew, molding it into a predetermined sectional form and extruding itfrom a nozzle attached to the end of the cylinder, wherein

the height of a flight on an upstream side of the screw is made lowerthan the height of a flight on a downstream side.

According to a second aspect of the present invention, there is provideda screw, wherein the height of the flight located below the hopper portof the screw is made lower than the height of the flight on a downstreamside.

According to a third aspect of the present invention, there is provideda screw, wherein the number of threads on an upstream side of the screwis made smaller than the number of threads on a downstream side.

According to a fourth aspect of the present invention, there is provideda screw, wherein the interval of the threads on an upstream side is madewider than the interval of the threads on a downstream side.

According to a fifth aspect of the present invention, there is provideda screw, wherein the diameters of the threads on an upstream side of thescrew are made larger than the diameters of the threads on a downstreamside.

According to a sixth aspect of the present invention, there is provideda screw, wherein the height of the flight located below the hopper portis made 2 to 6% smaller than the diameter of the screw.

According to a seventh aspect of the present invention, there isprovided a process for producing a screw for use in an extruder forcarrying a rubber material supplied from a hopper port at the rear ofthe cylinder of the extruder by the screw, molding it into apredetermined sectional form and extruding it from a nozzle attached tothe end of the cylinder, the process comprising cutting away aperipheral portion of the screw so that the height of a flight locatedbelow the hopper port of the screw for existing extruders becomes 2 to6% smaller than the diameter of the screw.

According to an eighth aspect of the present invention, there isprovided a process for producing a tire rubber member by using the screwaccording to any one of the first to sixth aspects.

According to a ninth aspect of the present invention, there is provideda tire rubber member manufactured by using the screw according to anyone of the first to sixth aspects and having a gauge fluctuation of 0.15mm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the constitution of an extruder according toan embodiment of the present invention;

FIGS. 2(A) and 2(B) are partially enlarged views of a screw for use inthe extruder according to the embodiment of the present invention;

FIG. 3 is a diagram showing the constitution of an extruder of the priorart;

FIG. 4 is a diagram showing the constitution of a screw for use in anextruder having a high delivery rate; and

FIG. 5 is a diagram for explaining the gauge fluctuation of an extrudedproduct.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be describedhereinunder with reference to the accompanying drawings.

FIG. 1 is a diagram of an extruder 10 having a screw for extrudersaccording to an embodiment of the present invention. Reference numeral11 denotes a cylinder as a container, 12 a screw which is rotatablyfitted in the cylinder 11, 13 a hopper for injecting a rubber materialheated with an unshown warming mill into the cylinder 11, 14 a headmounted to the end of the above cylinder 11, and 15 a nozzle attached tothe end of the head 14 and having an opening 15 a for molding thematerial into a predetermined required sectional form and detachablefrom the head 14 so that it can be exchanged according the type of anextruded product.

In this embodiment, a screw having a double-thread structure as a wholeand a single-thread structure only on the hopper port 13 s side near theinlet of the cylinder 11, in which a peripheral portion of a flightportion 12 z is smoothly cut away at an angle θ so that the height ofthe screw flight portion 12 z crossing the hopper port 13 s, that is,located below the hopper port 13 s becomes smaller by a predetermineddistance “d” than the diameter D of the screw 12 as shown in FIGS. 2(a)and 2(b) is used as the screw 12. More specifically, the above “d” isset to around 4% of the diameter D of the screw 12. The above θ is setto around 90° at maximum. The height of the flight portion 12 a on adownstream side is the same as the diameter D of the screw 12.

Since the return of the material occurs in the vicinity of the hopperport 13 s thereby, all the rubber material injected from the hopper port13 s is not forced into the cylinder 11. Therefore, a change in pressureapplied to the above flight portion 12 z in the vicinity of the hopperport 13 s is reduced and the extrusion pressure is made uniform.Accordingly, the delivery rate of the extruded product extruded from theopening 15 a of the nozzle 15 is made uniform, thereby making itpossible to reduce the gauge fluctuation of the extruded product.

When a tread was actually extruded and molded by using an extruderequipped with the above screw 12, the delivery rate was the same as thedelivery rate of a screw 62 for high-delivery type extruders shown inFIG. 4 and (about 20%) higher than that of the prior art and still thegauge fluctuation of the tread was half (0.15 mm) of the gaugefluctuation (0.3 mm) when the above screw 62 was used.

In this embodiment, since the height of the flight portion 12 z locatedbelow the hopper port 13 s of the above screw 12 for extruders whichcarries a rubber material supplied from the hopper port 13 s at the rearof the cylinder 11 of the extruder 10 is made lower than the height ofthe flight portion on a downstream side, the pulsation of the extrudedproduct can be reduced while a high delivery rate is maintained.

When a tire rubber member is manufactured by using the above screw 12,it is possible to manufacture a high-precision tire rubber member havinga gauge fluctuation of 0.15 mm or less.

In the above embodiment, the gauge fluctuation of the extruded productis reduced while the delivery rate is maintained by improving ahigh-delivery type screw 62 of the prior art. The present invention isdirected not only to the above screw 62 but also to a screw havinganother structure such as a single-thread structure as a whole.

In the above embodiment, the screw 12 having a double-thread structureas a whole and a single-thread structure only on the hopper port 13 sside is used. The thread structure of the screw is not limited to this.When a screw having a smaller number of threads on an upstream side thanthe number of threads on a downstream side, for example, two threads onan upstream side and three threads on a downstream side is used, thedelivery rate of the extruded product is made uniform and the gaugefluctuation of the extruded product can be reduced.

Since the delivery rate of the extruded product can be made furtheruniform by making the interval of threads on an upstream side of thescrew wider than the interval of threads on a downstream side or bymaking the diameters of the threads on an upstream side of the screwwider than the diameters of the threads on a downstream side, it ispossible to further reduce the gauge fluctuation of the extrudedproduct.

In the above embodiment, the peripheral portion of the flight portion 12z located below the hopper port 13 s of the screw 12 for existingextruders is cut away. When a screw for extruders is to be newlymanufactured, the height of a flight portion located below the hopperport 13 s of the screw is made smaller by around 4% than the diameter ofthe screw 12.

The height of the flight portion 12 z located below the hoper port 13 smay be made 2 to 6% smaller than the diameter of the screw. When theabove value “d” is smaller than 2%, the material rarely returns and thepulsation of the extruded product cannot be reduced fully. When thevalue is larger than 6%, the rubber material cannot be forced into thecylinder 11 smoothly, the delivery rate cannot be made uniform.

Industrial Feasibility

As described above, according to the present invention, since the heightof the flight located below the hopper port of the screw for extrudersis made lower than the height of the flight on a downstream side, theextrusion pressure and delivery pressure of the screw can be madeuniform and the gauge fluctuation of the extruded product can be reducedwhile a high delivery rate is maintained.

When a tire rubber member is manufactured by using the above screw, atire rubber member having an extremely small gauge fluctuation can beobtained.

1. A screw for use in an extruder for carrying a rubber materialsupplied from a hopper port at the rear of the cylinder of the extruderby the screw, molding it into a predetermined sectional form andextruding it from a nozzle attached to the end of the cylinder, whereinthe height of a flight on an upstream side of the screw is made lowerthan the height of a flight on a downstream side.
 2. The screw accordingto claim 1, wherein the height of the flight located below the hopperport of the screw is made lower than the height of the flight on adownstream side.
 3. The screw according to claim 1 or 2, wherein thenumber of threads on an upstream side of the screw is made smaller thanthe number of threads on a downstream side.
 4. The screw for use in anextruder according to claim 1 or 2, wherein the interval of the threadson an upstream side is made wider than the interval of the threads on adownstream side.
 5. The screw for use in an extruder according to claim1 or 2, wherein the diameters of the threads on an upstream side of thescrew are made larger than the diameters of the threads on a downstreamside.
 6. The screw for use in an extruder according to any one of claims1 to 5, wherein the height of the flight located below the hopper portis made 2 to 6% smaller than the diameter of the screw.
 7. A process forproducing a screw for extruders, comprising cutting away a peripheralportion of the screw so that the height of a flight located below thehopper port of the screw for existing extruders becomes 2 to 6% smallerthan the diameter of the screw.
 8. A process for producing a tire rubbermember by using the screw according to any one of claims 1 to
 6. 9. Atire rubber member manufactured by using the screw according to any oneof claims 1 to 6 and having a gauge fluctuation of 0.15 mm or less.